If your existing breaker panel has a second set of lugs
on the main input for going to a second panel, I would use
these. Be sure to use the same wire size that goes from the
meter to the main panel to go from the main to the second panel.
It isn't far, and won't cost much. Probably something like 2/0 , if
memory serves, and carry the neutral and ground and both
sides of the line. Do not use a new ground rod for the second
panel, but put a BIG neutral, like #6 or bigger, between the two
Use both phases of the input and try to put about half of
the breakers in the new panel on one line and the other half
on the other. They will all work on one phase, but dividing them
is a better way to do it.....